Method for the non-destructive testing of a casing by colorimetry

ABSTRACT

A method for non-destructively testing the heating of a determined zone of a part made of a polymer material, the method comprising the following steps: a) taking at least one colorimetric measurement on said determined zone to be tested and obtaining the value ap of the parameter a of the CIELAB colorimetric space; b) taking at least one colorimetric measurement on said reference zone of said part and obtaining the value ap/ref of the parameter a of the CIELAB colorimetric space; c) calculating Δap=ap−ap/ref; and d) establishing a risk of heating said determined zone to be tested if Δap is higher than a threshold value A1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent No.62/462,154, filed Feb. 22, 2017, and French Patent Application No.1751896, filed Mar. 8, 2017, the contents of each of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method for non-destructively testinga part comprising a matrix, for example, made of polymer. In particular,reinforcing fibres can be integrated to the matrix.

BACKGROUND

Conventionally, the upstream end of a turbine engine comprises a fancomprising a wheel formed of a plurality of blades surrounded on theoutside by an annular casing which can made of a metal material or of acomposite material comprising a matrix integrating reinforcing fibres,such as a polymer matrix, for example, epoxide polymer, and carbon fibreor glass fibre reinforcing fibres. This casing enables an initialcompression of air entering in the turbine engine and also ensures afunction for confining the blades in case of loss of one of them. Thefan casing is surrounded by a plurality of equipment supply ducts, inparticular, by a pressurised air supply duct, at a temperature of around200° C., an engine called APU (auxiliary power unit) used to start theturbojet as well as supplying the aircraft cabin with electricity whenthe aircraft is grounded.

In case of malfunctioning, such as a leak from the pressurised airsupply duct, the air can lead to a significant local heating of thecasing, since the temperature of the air is around 200° C. When thecasing is made of a metal material, for example, aluminium, the heatingis not impacted by the mechanical integrity of the casing. In the caseof a matrix fan case integrating the reinforcing fibres, the mechanicalstrength thereof following being heated, must be able to be guaranteed.

Therefore, it is understood that the non-destructive test of a compositematrix casing with reinforcing fibres is particularly important, and allthe more so, as a composite casing is proving to be very expensive.

It has thus been proposed to apply thermosensitive paints on the casing.However, the lifespan of these paints highly limits their interest,since an engine can be used for period longer than the lifespan of thesepaints, in particular for long- or medium-haul aircraft. Furthermore,during the placing of an engine, this conventionally undergoes cleaningby scraping, which causes a total removal of the layer ofthermosensitive paint, involving another step of applying a layer ofpaint. Finally, if a thermosensitive paint enables to visually considerthe heating state of a given zone of a fan case, it only proves to be anindirect measurement of the state of the internal structure of thecasing and does not enable a specific quantification of the internalstructure of the casing.

SUMMARY

The invention, in particular, aims to provide a simple, effective andeconomic solution to the problems of the prior art defined above.

To this end, it proposes a method for non-destructively testing theheating of a zone determined to be tested of a part made of a polymermaterial, the method comprising the following steps:

-   -   a) taking at least one colorimetric measurement on said        determined zone to be tested and obtaining the value a_(p) of        the parameter a of the CIELAB colorimetric space,    -   b) taking at least one colorimetric measurement on said        reference zone of the part and obtaining the value a_(p/ref) of        the parameter a of the CIELAB colorimetric space,    -   c) calculating Δa_(p)=a_(p)−a_(p/ref),    -   d) establishing a risk of heating said determined zone to be        tested if Δa_(p) is higher than a threshold value A1.

The method according to the invention enables to determine if theexamined part has been subjected (or not) to an overheating beyond anacceptable limit. In practice, if a risk of overheating is established,a physicochemical analysis of the part is carried out so as todetermine, more specifically, the level of overheating. However, aphysicochemical analysis must be carried out in a laboratory, whichrequires a disassembly of the part from the mechanical unit whereon itis assembled. Thus, when the part to be tested is a turbine enginecasing, the laboratory test involves a placing of the aircraft engineand highly increases the immobilisation time of the aircraft andconsequently the operating costs.

The invention proposes to establish a risk of heating a given zone of apart, compared with a supposedly normal reference section, in otherwords, a section that has not been subjected to heating, of the examinedpart, which enables to establish a reference on the part itself, inorder to consider the normal variations of the parameter a due to theoutside environment wherein the part has been since it was manufactured.The term “normal” refers to the standard conditions of use of the part,enabling to have a predetermined lifespan of the part.

In the CIELAB colorimetric space, the variation in the value of theparameter a gives information on the level of overheating subjected toby the determined zone to be tested of the part.

The threshold A1 can be determined by using a reference databasecomprising colorimetric measurements taken on a plurality of referencesamples made of a polymer material, in particular, made of reinforcingfibres, having been subjected to a determined temperature for apredetermined time period. From a plurality of measurements of theparameter a, a threshold A1 is established, below which the mechanicalintegrity of the part cannot be guaranteed without a physicochemicalanalysis.

The value Δa_(p) gives information on the (component) level of redcolour present in the given zone which is tested.

In the application, the CIELAB colorimetric space means the CIE L*a*b*system or the acronym CIE means the Commission Internationale del'Eclairage (International Lighting Commission). The asterixis havevoluntarily been omitted in the text to avoid encumbering the notations.

In the present document, the CIELAB space corresponds to that defined bythe standard NF EN ISO 11664-4 (2011-07-01), of which the title is“Colorimétrie—Partie 4: espace chromatique L*a*b* CIE 1976”(“Colorimetry—Part 4: CIE L*a*b* colour space 1976”).

The method can further comprise the following steps:

-   -   obtaining the value b_(p) of the parameter b and the value L_(p)        of the parameter L of said at least one colorimetric measurement        taken in step a),    -   obtaining the value b_(p/ref) of the parameter b and the value        L_(p/ref) of the parameter L of said at least one colorimetric        measurement taken in step b),    -   calculating Δb_(p)=b_(p)−b_(p/ref) and calculating        ΔL_(p)=L_(p)−L_(p/ref),    -   establishing a risk of heating said determined zone to be        tested, if all the following conditions are checked:        -   Δa_(p) is higher than a threshold value A2, A2 being lower            than A1,        -   Δb_(p) is higher than a threshold value B1,        -   ΔL_(p) is lower than a threshold value L1.

The value Δb_(p) gives information on the level of yellowing of thedetermined zone which is tested. It is reminded that, in this colourspace, the value of the parameter b develops from negative values up topositive values, in other words, from blue to yellow. The variation invalue of the parameter L gives information on the development of theluminance/clarity and develops from the value 0, corresponding to black,up to the value 100, corresponding to white. The threshold L1 enables togive information on the decrease in clarity of the tested zone, inrelation to the reference zone.

Thus, if during step d), the risk of overheating is not established, themethod consists of making three successive comparisons, of which thepositive results imply the establishment of a risk.

According to a characteristic of the invention, the thresholds A2, B1,L1 are determined by using said colorimetric measurements stored in thereference database.

According to another characteristic of the invention, establishing thedatabase comprises the following steps:

-   -   for each first sample, obtaining the value a′ of the parameter a        of the CIELAB colorimetric space, from at least one colorimetric        measurement,    -   for each first sample, calculating Δa′=a′−a′_(ref), where:        -   α′_(ref) corresponds to the value of the parameter a of the            CIELAB colorimetric space, this value having been obtained            on a second sample made of a polymer material, in            particular, made of reinforcing fibres, having the same time            of existence as said first sample considered and having been            kept at a temperature within a range of temperatures, such            as that of preserving the mechanical integrity of the second            sample, or such as a range between 0 and 40° C., which could            further consider the exposure to rays in the ultraviolet            field,    -   carrying out a test to determine the mechanical properties of        each one of the first samples,    -   determining the threshold A1 from comparing the data from the        tests carried out in the previous step and the values Δa′        contained in the database.

Establishing the database also comprises the following steps:

-   -   for each first sample, obtaining the values L′ and b′ of the        respective parameters L and b of the CIELAB colorimetric space,        from at least one colorimetric measurement,    -   for each first sample, calculating Δb′=b′−b′_(ref) and        ΔL′=L′−L′_(ref), where:        -   b′_(ref) and L′_(ref) respectively correspond to the values            of the parameters b and L of the CIELAB colorimetric space,            these values having been obtained on a second sample made of            a polymer material, in particular, made of reinforcing            fibres, having the same time of existence as said first            sample considered and having been kept at a temperature            within a range of temperatures, such as that of preserving            the mechanical integrity of the second sample, or such as a            range between 0 and 40° C., which could further consider the            exposure to rays in the ultraviolet field,    -   determining the thresholds A2, B1 and L1 from comparing the data        from said tests and the values Δa′, Δb′ and ΔL′ contained in the        database.

According to the invention, testing the mechanical properties of thefirst samples can comprise at least one step for mechanically testingthe part, for example, by traction and/or compression.

The method can again comprise the following steps:

-   -   calculating the colour difference ΔE_(p) between the        colorimetric measurements obtained in steps a) and b),    -   from the value ΔE_(p) obtained in the previous step, determining        the time period during which said zone to be tested of the part        has been subjected to a determined heating temperature and        determining said heating temperature, by using a reference        database comprising the values ΔE′ of colour differences        obtained from a plurality of samples made of a polymer material        having been subjected to a determined temperature for a        determined time period.

The value ΔE_(p) is determined for said zone to be tested by making thecalculation ΔE_(p)=√{square root over (ΔL_(p) ²+Δa_(p) ²+Δb_(p) ²)}.

The values ΔE′ are determined for each first sample by making thecalculation ΔE′=√{square root over (ΔL′²+Δa′²+Δb′²)}.

To limit errors in measurement and to average out the experimentalvariability, each value considered of the parameters L, a, b of theCIELAB colorimetric space can be obtained by taking the average of atleast five successive colorimetric measurements at the place considered.

According to the invention, in case a heating risk is established, themethod can also further comprise a subsequent step to step d), ofcarrying out a physicochemical analysis of the determined zone to betested of the part, so as to determine on the state of damage of saiddetermined zone.

The part analysed can be made of a polymer material comprisingreinforcing fibres.

Preferably, a step of cleaning the surface whereon a colorimetricmeasurement is intended to be taken, is carried out prior to saidcolorimetric measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be best understood, and other details, advantages andcharacteristics of the invention will appear upon reading the followingdescription, made as a non-exhaustive example, in reference to thefollowing figures:

FIG. 1 is a schematic view of a turbine engine fan case to be tested;

FIG. 2 is a representative view of the CIELAB colorimetric space axesused in the present document;

FIG. 3 is a schematic view of a plurality of samples, each havingundergone oxidation for a given time (vertically) at a given temperature(horizontally);

FIG. 4 is a schematic view illustrating a line of separation between afirst zone 1 and a second zone 2;

FIG. 5 is a schematic view of a graph where each point represents asample from FIG. 3 having been subjected to a vitreous transitiontemperature of the tested material, each point being placed on the graphaccording to the values of the parameters a and b, thereof, a beingrepresented on the x-axis and b being represented on the y-axis;

FIG. 6 is a schematic view of a graph where each point represents asample from FIG. 3 having been subjected to a higher temperature thanthe vitreous transition temperature of the polymer material, each pointbeing placed on the graph according to the values of the parameters aand b, thereof, a being represented on the x-axis and b beingrepresented on the y-axis;

FIG. 7 is a flowchart of the functioning of the decision-making methodduring a step of non-destructive testing on a given zone to be tested,of a determined part such as the casing from FIG. 1;

FIG. 8 is a graph representing the development in the colour differenceover time for the samples from FIG. 3;

FIG. 9 is a larger-scale graph of a given zone of the graph from FIG. 8.

DETAILED DESCRIPTION

As explained above, the fan casing 10 represented in FIG. 1, made ofpolymer, in particular of reinforcing fibres, can undergo when switchedon, a local heating that should be able to be characterised by anon-destructive method enabling to determine the state of the casing 10in order to determine if it can (or not) be kept in service in a turbineengine.

FIG. 2 represents the CIELAB space used in the present document foranalysing colorimetric data obtained on the part to be tested, as wellas for establishing the reference database on a plurality of firstsamples. The CIELAB space is a system which enables to represent thetrichromatic components along three orthogonal axes between them. Theaxis L (or L*) represents the luminance or clarity axis (perfect black:L=0; perfect white: L=100). The axis a (or a*) represents the axis goingfrom green (negative values of a) to red (positive values of a). Theaxis b (or b*) represents the axis going from blue (negative values ofb) to yellow (positive values of b).

It is again reminded, that the space used is the CIE L*a*b*colorimetricspace, and that the asterixis have voluntarily been removed as this isusual.

In this colour system, a colour difference between a first colour ofcoordinates L₁, a₁, b₁ and a second colour of coordinates L₂, a₂, b₂ iscalculated as follows:

${{\Delta\; E} = \sqrt{{\Delta\; L^{2}} + {\Delta\; a^{2}} + {\Delta\; b^{2}}}},{where}$Δ L = (L₁ − L₂) Δ a = (a₁ − a₂) Δ b = (b₁ − b₂)

This method of calculating the colour difference is the one used later,as this will appear following the description.

The invention proposes to establish a reference database comprisingcolorimetric measurements according to the CIELAB colour system. Theterm “reference” used below is to be understood as meaning an element ofthe reference database comprising colorimetric measurements and moregenerally, data obtained from the reference samples.

For this, a batch of a plurality of first samples 12 of a materialsimilar to the part to be analysed is constituted. FIG. 3 illustratessuch a batch which thus comprises several first samples 12 of fan case10 made of a polymer material, preferably of reinforcing fibres,arranged in the form of lines and columns. Along a given line, eachfirst sample 12 is subjected to a given temperature, of which theexposure time is given by the position along a line. Of course, thedatabase should comprise a number of first samples 12 enabling torealise different levels of thermal oxidation, that the polymer can besubjected to under actual functioning conditions. Thus, the databaseshould comprise the first samples 12 having been subjected to theaforementioned temperatures for periods of time going up to at least 6months.

In the configuration represented as an example, the first samples 12have been subjected to temperatures in ° C. of 120, 140, 150, 160, 180,200, 220 and 240° C. for periods in hours, spread out from 1 up to 1440hours, which corresponds to a period of 2 months. The sample 14positioned in the lower left-hand corner in FIG. 1 represents a sample12 not having been subjected to any heating, which therefore constitutesthe absolute reference of a fan case 10 without any thermal heating. Itis observed in FIG. 1, that the first samples 12 darken as soon as thetemperature increases, and that the exposure time to a temperatureincreases, which is consistent with a thermal oxidation of the polymer.

FIG. 4 schematically represents FIG. 3 and comprises a line ofseparation 16 of a first zone 1 and a second zone 2. The first zone 1corresponds to the first samples 12 which have been subjected to anacceptable temperature for an acceptable time period, whereas the secondzone 2 corresponds to the first samples 12 which have been subjected totoo-high temperature for a given time period. Thus, if visually, it ispossible to establish this line of separation, it seems necessary toestablish one or several parameters enabling to objectively realise thestate of an analysed part. That is what is defined below by establishingthe reference database.

Establishing the reference database first consists of taking acolorimetric measurement for each one of the first samples and bydeducing the values L′, a′ and b′ of the parameters L, a, b of theCIELAB colorimetric space, from at least one colorimetric measurement.

It must first be noted, that the values of the parameters L, a, b can beobtained from several colorimetric measurements in each zone where themeasurement is taken, in other words, to establish the database or whenit is wanted to test a determined zone of a part, as this will beexplained later.

Now reference is made to FIGS. 5 and 6, each representing a graph whereeach point represents a first sample from FIG. 3, each point beingplaced on the graph according to the values of the parameters a and bthereof, a being represented on the x-axis and being represented on they-axis. In FIG. 5, the first samples have been subjected to atemperature lower than the vitreous transition temperature of thepolymer material, here 170° C., and in FIG. 6, the first samples havebeen subjected to a temperature higher than said vitreous transitiontemperature.

In FIG. 5, it has been observed that a group 18 of points is found atthe values of the parameter having a value of less than zero whereas, inFIG. 6, for the first samples having been subjected to a temperaturehigher than 170° C., it has been observed that a group 20 of values ofthe parameter a are higher than zero. Consequently, it is possible todiscriminate on the thermal state, in other words, the heating of agiven zone of a part, from the measurement of this parameter. It will benoted that in FIG. 6, a second group 22 has values of the parameter awhich are less than zero, but these points correspond to very lowexposure times, less than 10 hours, which are not to be considered. Onthe graph in FIG. 5, it has also been observed that the variation ismade mainly along the axis b, this variation enabling to highlight theyellowing by natural ageing over time of the polymer and of the resin inthe case of a fan casing. This variation along the axis b is alsovisible in FIG. 6.

Thus, it is understood that it is possible, with a colorimetricmeasurement in the CIELAB space, to differentiate between the naturalageing of the fan case and an accidental overheating of by comparingwith a reference database.

To each first sample 12, it is calculated that Δa′=a′−a′_(ref),Δb′=b′−b′_(ref) and ΔL′=L′−L′_(ref), where:

-   -   a′_(ref), b′_(ref) et L′_(ref) respectively correspond to the        values of the parameters a, b, L of the CIELAB colorimetric        space, these values having been obtained on a second sample 14        made of a polymer material, in particular, of reinforcing        fibres, having the same time of existence as said first sample        considered, and having been kept at a temperature within a range        of temperatures, such as that of preserving the mechanical        integrity of the second sample 14, or such as a range between 0        and 40° C., which could further consider the exposure to rays in        the ultraviolet field.

The colorimetric measurement on the second sample 14 associated witheach measurement of a first sample 12 can be taken with the referencesample 14 that has been observed under the conditions stated in theprevious paragraph.

For each first sample 12, a test is then carried out, aiming todetermine the mechanical properties thereof, in order to determine theability or not thereof to constitute a sample that can be used under thedetermined conditions. Thus, it is determined if the heating subjectedby each first sample makes it useable or not. The test carried out cancomprise at least one step for mechanically testing the part, forexample, by traction and/or compression.

Finally, a comparison of the data from the tests with the values Δa′,Δb′ et ΔL′ contained in the reference database is made, in order toestablish the thresholds A1, A2, B1 and L1. The threshold A1 correspondsto a threshold beyond which it is considered that the fan case 10 mustbe arranged to be subjected to a more in-depth inspection of the zone tobe tested (FIG. 7).

To carry out a non-destructive test operation on the casing 10 from FIG.1, first, the following steps are carried out, schematised in FIG. 7:

-   -   a) taking at least one colorimetric measurement on said        determined zone 24 to be tested of the casing 10 and obtaining        the value a_(p) of the parameter a of the CIELAB colorimetric        space,    -   b) taking at least one colorimetric measurement on a reference        zone 26 of the casing 10 and obtaining the value a_(p/ref) of        the parameter a of the CIELAB colorimetric space,    -   c) calculating Δa_(p)=a_(p)−a_(p/ref),    -   d) establishing a risk of heating said determined zone to be        tested if Δa_(p) is higher than a threshold value A1.

The reference zone 26 of the casing 10 is a zone which has not sufferedthermal damage.

In the case where the value Δa_(p) is lower, a second step is carriedout, aiming to determine if the part must undergo (or not) a test allthe same, this time considering the value b_(p) of the parameter b andthe value L_(p) of the parameter L obtained from the colorimetricmeasurement on the zone 24 to be tested of the casing, as well as thevalue b_(p/ref) of the parameter b and the value L_(p/ref) of theparameter L obtained from the colorimetric measurement on the referencezone 26 of the casing.

The method then consists of calculating Δb_(p)=b_(p)−b_(p/ref) andcalculating ΔL_(p)=L_(p)−L_(p/ref), and establishing a risk of heatingsaid determined zone 24 to be tested if all the following conditions arechecked:

-   -   Δa_(p) is higher than a threshold value A2, A2 being lower than        A1,    -   Δb_(p) is higher than a threshold value B1,    -   ΔL_(p) is lower than a threshold value L1.

In this case, it is looked to determine if the tested zone 24 has agreater yellowing than the threshold value B1, if the clarity ΔL_(p) islow, in other words, lower than the threshold L1 while having a Δa_(p)higher than a threshold value A2 lower than A1.

If one of the conditions above is not checked, the zone 24 to be testedis considered as not being damaged and the casing 10 can be used.

The non-destructive testing by colorimetry operation can be carried outunder the wing of the aircraft, which enables to have a quick andreliable decision regarding the placing or not of the casing and reducesthe unnecessary maintenance operations.

The parameters A1, A2, B1, L1 must be established to each type of part10 and are therefore connected to the material of said part and alsodepend on the colorimetric measuring device.

Thus, in an example of taking colorimetric measurements with a KonicaMinolta CM700d colorimeter, A1 is equal to 4.3, A2 is equal to −1, B1 isequal to 12.6 and L1 is equal to −0.9.

From the aforementioned colorimetric measurements contained indatabases, it is possible to determine the time period during which thetested zone has been subjected to determined temperature, as well asthis temperature.

For this, the colour difference ΔE_(p)=√{square root over (ΔL_(p)²+Δa_(p) ²+Δb_(p) ²)} is calculated for the tested zone by using thecolorimetric measurements obtained on the tested zone and on thereference zone of the analysed casing. For each one of the firstsamples, the colour difference ΔE′=√{square root over (ΔL′²+Δa′²+Δb′²)}is also calculated from the values ΔL′, Δa′ and Δb′. From the valuesΔE′, it is possible to trace the development of ΔE′ over time forseveral determined temperatures as this is represented in FIGS. 8 and 9.

FIG. 8 represents the development of ΔE′ over time for the temperatures120, 140, 150 and 160° C., these curves are respectively referencedΔE′₁₂₀, ΔE′₁₄₀, ΔE′₁₅₀ and ΔE′₁₆₀.

To avoid the colour difference development curves being impacted by thenon-relevant values of ΔE′ because of a very clear colouring or acolouring that is too dark, the points associated with such values ΔE′are removed. A very clear colouring can be due to an exposure to a lowtemperature for a relatively short time period. This zone corresponds tothe zone 28 in FIG. 3 and cannot validly be considered in thecolorimetric analysis. A too dark colouring can be due to an exposure toa too high temperature for a significant time period. This zonecorresponds to the zone 30 in FIG. 3 and cannot validly be considered inthe colorimetric analysis.

Therefore, the situation is limited to the relatively long time ofexposure, longer than 300 hours, as this is represented in FIG. 9. Withthis graph, it is possible from a measurement ΔE_(p) obtained on a givenzone of a casing, to determine the temperature of exposure by tracingthe constant ordinate line ΔE_(p) and searching for the interceptedcurve which therefore indicates the temperature subjected to by saidtested zone and the exposure temperature represented by the abscissa ofthe intersection point.

The invention claimed is:
 1. A method for non-destructively testing aheating of a determined zone to be tested of a part made of a polymermaterial, the method comprising the following steps: a) taking at leastone colorimetric measurement on said determined zone to be tested andobtaining a value a_(p) of a parameter a of a CIELAB colorimetric space,b) taking at least one colorimetric measurement on a reference zone ofsaid part and obtaining a value a_(p/ref) of the parameter a of theCIELAB colorimetric space, c) calculating Δa_(p)=a_(p)−a_(p/ref),obtaining a value b_(p) of a parameter b and a value L_(p) of aparameter L of said at least one colorimetric measurement taken in stepa), obtaining a value b_(p/ref) of the parameter b and a value L_(p/ref)of the parameter L of said at least one colorimetric measurement takenin step b), calculating Δb_(p)=b_(p)−b_(p/ref) and calculatingΔL_(p)=L_(p)−L_(p/ref), and d) establishing a risk of overheating saiddetermined zone to be tested if: Δa_(p) is higher than a threshold valueA1, Δa_(p) is higher than a threshold value A2, A2 being lower than A1,Δb_(p) is higher than a threshold value B1, ΔL_(p) is lower than athreshold value L1.
 2. The method according to claim 1, wherein thethreshold A1 is determined by using a reference database comprisingcolorimetric measurements taken on a plurality of first samples made ofa polymer material, in particular, made of reinforcing fibres, havingbeen subjected to a determined temperature for a determined time period.3. The method according to claim 2, wherein the thresholds A2, B1, L1are determined by using said colorimetric measurements stored in thereference database.
 4. The method according to claim 3, whereinestablishing the database comprises the following steps: for each firstsample, obtaining a value a′ of the parameter a of the CIELABcolorimetric space, from at least one colorimetric measurement, for eachfirst sample, calculating Δa′=a′−a′_(ref), where: a′_(ref) correspondsto a value of the parameter a of the CIELAB colorimetric space, thevalue having been obtained on a second sample made of reinforcingfibres, having a same time of existence as said first sample consideredand having been kept at a temperature within a range of temperatures topreserve a mechanical integrity of the second sample, carrying out atest to determine mechanical properties of each one of the firstsamples, determining the threshold A1 from comparing data from the testscarried out in previous steps and the values Δa′ contained in thedatabase.
 5. The method according to claim 4, wherein establishing thedatabase also comprises the following steps: for each first sample,obtaining values L′ and b′ of the parameters L and b of the CIELABcolorimetric space, from at least one colorimetric measurement, for eachfirst sample, calculating Δb′=b′−b′_(ref) and ΔL′=L′−L′_(ref), where:b′_(ref) and L′_(ref) respectively correspond to values of theparameters b and L of the CIELAB colorimetric space, these values havingbeen obtained on a second sample made of reinforcing fibres, having asame time of existence as said first sample considered and having beenkept at a temperature within a range of temperatures to preserve amechanical integrity of the second sample, determining the thresholdsA2, B1 and L1 from comparing data from said tests and the values Δa′,Δb′ and ΔL′ contained in the database.
 6. The method according to claim5, wherein the range of temperatures is a range between 0 and 40° C. 7.The method according to claim 4, wherein said test comprises at leastone step for mechanically testing the part.
 8. The method according toclaim 7, wherein the at least one step for mechanically testing the partcomprises at least one of traction and compression.
 9. The methodaccording to claim 4, wherein the range of temperatures is a rangebetween 0 and 40° C.
 10. The method according to claim 1, furthercomprising the following steps: calculating a color difference valueΔE_(p) between the colorimetric measurements obtained in steps a) andb), from the value ΔE_(p) obtained in the previous step, determining atime period during which said zone to be tested of the part has beensubjected to a determined heating temperature and determining saidheating temperature, by using a reference database comprising values ΔE′of color differences obtained from a plurality of samples made of apolymer material having been subjected to a determined temperature for adetermined time period.
 11. The method according to claim 1, whereineach value considered of the parameters L, a, b of the CIELABcolorimetric space is obtained by taking an average of at least fivesuccessive colorimetric measurements at a place considered.
 12. Themethod according to claim 1, wherein in case of establishing a risk ofoverheating, the method further comprises a subsequent step to step d)for carrying out a physicochemical analysis of the determined zone to betested of the part so as to determine a state of thermal damage of saiddetermined zone to be tested.
 13. The method according to claim 1,wherein the polymer material comprises reinforcing fibres.
 14. Themethod according to claim 1, further comprising a step of cleaning asurface whereon a colorimetric measurement is intended to be taken.